This invention relates to the protection of d-c electrical loads from damaging overcurrent. In particular it applies to circuits where a breakdown of insulation in the load causes a sudden large increase in current. Such an increase is characteristic of a fault in a high-voltage tube or a high-voltage accelerating structure.
Electrical loads that are connected to high-voltage d-c power supplies are frequently subject to a risk of destruction by delivery of some of the power from the supply to portions of the load in excessive quantities as a result of a fault. This is especially true of high-voltage accelerating structures connected to high-voltage power supplies, although almost any load that is connected to a d-c power supply is capable of being damaged, at least locally, by fault currents. However, the threat of electrical breakdown is greatest in those electrical loads that are connected to power supplies at relatively high voltage, of the order of kilovolts. It is customary to protect such loads from damage by the use of a crowbar. This is a controllable short circuit that can be placed across the output terminals of the power supply to short out the load and thereby bypass the electrical energy that would otherwise be delivered in an excessive amount to the load. The crowbar is often a gas tube such as an ignitron, a solid-state device such as a silicon-controlled rectifier (SCR), or a triggered spark gap. A control signal for the crowbar is derived from a sensor of load current, load voltage or both, and the presence of a damaging fault is caused to generate a signal that operates the crowbar. With the output of the supply shorted, the load is relieved of the excess electrical energy that would otherwise have been delivered to it and the overcurrent protective mechanisms normally built into the circuit will trip to prevent damage to the power supply.
The arrangement just described has one major disadvantage. This is the fact that the signal required to operate the crowbar results from the application of damaging currents to the load. It frequently happens that the load is damaged before the crowbar can be applied. Even in circumstances when the load is not immediately damaged by the excessive current, it remains true that the load is caused to carry current beyond its normal operating range in order to trip the crowbar. A fault current that may be as much as several orders of magnitude larger than normal load currents thus passes for a period of time through the load. It would clearly be desirable to have a protective mechanism whose operation did not require damaging currents.
It is an object of the present invention to provide a better means of protecting d-c electrical loads from damage due to overcurrents.
It is a further object of the present invention to provide a means of overcurrent protection for d-c electrical loads that does not require the passage of damaging currents through the load to operate the protection.
Other objects will become apparent in the course of a detailed description of the invention.